1. Field of the Invention
The present invention relates to a surface treatment process for forming a thin film on the surface of a substrate to provide a hard surface layer.
2. Description of the Prior Art
In order to obtain hard layers on the surfaces of substrates, there have been proposed a PVD (physical vapor deposition) process such as an ion plating process and a CVD (chemical vapor deposition) process employing chemical reaction under a high temperature. These processes are adapted to form hard layers of carbide, nitride, oxide, boride or the like of several microns in thickness on the surfaces of substrates. The hard layers thus formed different thickness in different spots on the substrate, and hence said processes cannot be applied where a layer having an accurate thickness is required.
The CVD process is adapted to form a thin film under a considerably high temperature, whereby adhesion is obtained finely to some extent between the substrate and the surface-coating film. By the CVD process, however, an brittle layer is formed between the substrate and the surface-coating film, thereby to reduce strength of the adhesion to the substrate. On the other hand, the ion plating process is adapted to form a hard layer under a relatively low temperature in comparison with the CVD process. However, the adhesion between the substrate and the surface-coating film obtained by the ion plating process is not sufficient.
A process of injecting a nonmetal ion into a substrate as a surface treatment process has been proposed for forming a hard layer. However, such injecting process cannot be applied to all types of substrates, and application thereof is limited to those materials which react with a nonmetal ion to be hardened. For example, the surface layer is hardened by injecting a nitrogen ion into an Fe-alloy, whereas no surface layer is hardened by injecting the nitrogen ion into a Co-alloy or Ni-alloy.
In order to overcome the aforementioned problem, the inventors have proposed an improved surface treatment process in Japanese patent application No. 64507/1982 (patent laying-open gazette No. 181864/1983) filed on Apr. 16, 1982. This surface treatment process is characterized in that the nitrogen ion is injected into a substrate in a metal vapor atmosphere provided around the substrate to be treated. When, for example, Ti is employed as an atmospheric metal in this process, Ti is also injected with the nitrogen ion into the surface of the substrate. Thus, Ti and the nitrogen ion partially react with each other to generate strong nuclei of TiN on the surface, thereby to facilitate hardening of the surface. However, there is yet room for improvement in this process. Namely, Ti and N in the surface-coating film are preferably provided in the form of TiN coupled substantially in the ratio of 1:1 in order to improve adhesion between the substrate and the surface-coating film and increase hardness of the surface-coating film. However, the rate of Ti, which serves as the atmospheric metal, is inevitably increased in the aforementioned process combining vapor deposition and ion injection.